1. Field of the Invention
The invention relates to inclination sensing instruments and more particularly, to an instrument that provides variable capacitance as a function of inclination angle.
2. Description of the Prior Art
A carpenter's level comprising a transparent tube partially filled with a fluid, such that an air bubble exists within the fluid, has long been employed for determining deviations from the horizontal. These devices in the simplest forms, though inexpensive, do not furnish accurate measures of inclination angle but merely provide an indication of the deviation from the horizontal of a reference surface. For many applications such indications are adequate, while for others accurate measures of the inclination angles are necessary.
Inclination sensing instruments, based on the bubble in a liquid concept, that utilize capacitor variations caused by variable positions of the bubble have been employed to determine deviations from the horizontal. One such device has three, electrically isolated, aluminum foil electrodes covering the liquid containing vial. One electrode is a reference electrode that covers the entire outside of the vial except for the surface over the bubble path. The other two electrodes, each as long and as wide as the bubble, are symmetrically positioned over the bubble path on either side of the vial center. The liquid in the vial has a higher dielectric constant relative to that of the air bubble, thereby establishing a difference in capacitance between the capacitance formed by one sensing electrode and the reference electrode and the capacitance formed by the other sensing electrode and the reference electrode that is a function of the position of the bubble. This device exhibits a linearity and an accuracy that is a function of the location of the sensing electrodes on the vial and the bubble size, which may vary with the fluid temperature.
Other tilt indicating devices of the prior art utilize gravity type potentiometers. One such apparatus comprises a resistance wires axially positioned in a V shape or circular tube containing a conducting fluid extending to a given level above the lowest point of the tube. Two equal end segments of resistance wire, extending from the conducting fluid to each end, are formed when the device is coupled to a horizontally positioned referenced surface, thus providing wire segments of equal resistance. Tilting this reference surface from a horizontal orientation causes the tube to rotate, establishing wire segments between the conducting fluid and the ends that are of unequal length and resistance. The accuracy of these devices depend on the tolerances maintained on the resistance of the wire, while the stability is a function of the temperature coefficient of resistivity of the wire and temperature coefficient of expansion of the fluid.
The present invention provides a reliable inclinometer comprised of two capacitors having differential capacitor variations that are linear with tilt angle.